In 2007, Rick Kinder was working for a contractor, building a house in southern Colorado. The workers had just finished putting in all the doors, windows and sealing the house. Kinder and a colleague were working in the crawlspace, hanging insulation.

“And we just heard this big roar and then a big boom, and it threw us against the walls, and it just blew the whole top of the roof off,” Kinder says.

In 2007, Rick Kinder was building this house in Trinidad, Colo., when it blew up. An abandoned gas well leaking methane was underneath the house.

Colorado Oil and Gas Conservation Commission

He and his colleagues didn’t know it, but they were building on top of an abandoned gas well that was leaking methane — an odorless and highly explosive gas. No one was killed in the explosion, but the blast sent Kinder into cardiac arrest. He ended up having a quadruple bypass.

“And I lost probably a third of my heart, the muscle,” he says.

In many parts of the country, areas that are now full of houses and schools and shopping centers were once oil and gas fields. You wouldn’t know it by looking, but hidden underground, there are millions of abandoned wells.

New development happening on top of those old wells can create a dangerous situation.

No Systematic Monitoring Of Leaks

In most states, there is no requirement for homeowners to be notified about abandoned oil and gas wells on their properties.

Jeff Parsek bought his house in a neighborhood of cul-de-sacs and suburban ranch homes on the south side of Fort Collins, Colo., in 2004. He had no idea that state records show there is an abandoned well in his driveway until he was asked by NPR.

But unlike other homeowners who responded with curses and slammed doors, Parsek was unfazed by the news. “If it started to emit something, then I might [be worried], but to this point I’m not concerned,” he says.

The trouble is that it might be hard to know if the wells were emitting something. When a well stops producing commercial quantities of oil and gas, companies “abandon” it, usually by filling the well with cement to stop the flow of gas and fluids. The industry considers that the end of a well’s life.

“It’s not rocket science to plug these wells,” says Mark Watson, Wyoming’s oil and gas supervisor. “It’s a hole in the ground that’s pretty deep. You set cement, and cement lasts a long time.”

The belief that a well is dead once it’s plugged means there is no systematic monitoring for leaks. We simply don’t know what percent of abandoned wells are leaking — but we do know that at least in a handful of cases, it’s happened, as it did to Kinder.

Locations Of Old Wells Poorly Mapped

Kinder had worked in the oil and gas industry for nearly 30 years. If anyone were going to recognize an old well site, it would be him.

“There were no signs, no nothing there that would give us an indication that somebody had built there or had done some work,” he says.

Three maps by Inside Energy explore the locations of abandoned wells in Colorado and the expansion of residential areas into historical oil and gas fields.

Across the U.S., from Pennsylvania to Texas to California, there are millions of abandoned wells. But their locations are either unmapped or poorly mapped. Rob Jackson, a researcher at Stanford University, says keeping track of them is a low priority.

“When a state sees a well is plugged, they typically put a check mark by that well in a database, or in a file somewhere, and they don’t do anything for the most part,” he says.

Unless a well starts leaking fluids or a house blows up, the assumption is that everything is fine. Jackson says it’s partly an issue of states not having the resources to monitor the wells.

“But I also think the states aren’t that interested in some cases, in many cases, in the data,” he says. “I’m not sure that they really want to know.”

A No-Build-Zone Around Old Wells

In the Canadian province of Alberta, it’s a different story. Theresa Watson, an engineering consultant and former Alberta energy regulator, started pushing for better tracking, monitoring and regulation of abandoned wells back in the early 2000s. That was when people started to move into rural areas that were once oil and gas fields.

“If you don’t measure it, you don’t know what kind of risk you have,” Watson says. “I mean, ignorance is bliss, I guess, but I think most people will tell you that they’d want to know.”

Watson recommended not only monitoring abandoned wells near homes, but also outright prohibiting construction on top of them.

“From the public safety perspective, even a slow leak into a building can cause an explosion hazard,” she says.

Alberta now has a 15-foot no-build zone around abandoned wells. Similar rules are lacking in most of the U.S., even as new development is encroaching on old oil and gas fields in places like the Front Range of Colorado.

State oil and gas regulators say the situation is out of their hands.

“We really try to discourage the construction on top of old wells,” says Stuart Ellsworth, engineering manager for the Colorado Oil and Gas Conservation Commission.

But he says any actual prohibition is up to local governments that permit development. In turn, many of those local governments see abandoned wells as the state’s problem.

For those who can potentially be affected by these wells, just having rules that will protect them is a priority. Kinder says he doesn’t care who ends up making rules about building on top of abandoned wells — he just wants somebody to do it.

“Somebody needs to be held accountable for it,” he says. “To me, the most important thing is it doesn’t happen to somebody else. It doesn’t need to happen to somebody else.”

On May 4, workers dismantle the charred remains of a house in Firestone, Colo., where an unrefined gas line leak explosion killed two people in April. Brennan Linsley/AP

A few weeks ago Julia Chapman’s daughter was heading to a playdate across the street in their recently built suburb in Firestone, Colo. Suddenly, the friend’s house exploded, killing two of the friend’s relatives who were in the basement.

“It shook our home,” Chapman says. “We came out and we saw that it was essentially just collapsed on itself. The insulation was still floating in the air, down the street.”

It turned out a pipe from a nearby oil and gas well had been abandoned, but not properly sealed. The April 17 tragedy just north of Denver is prompting a lot of questions about how wells are regulated, something Chapman says she never thought about when she bought her house.

“We just sort of trusted that the city and the oil and gas knew what they were doing,” she says.

Colorado’s chief oil and gas regulator, Matt Lepore, called the explosion “highly unusual,” but the state is now reviewing its oil and gas operations.

The destroyed home was built 178 feet from an existing, dormant well. Anadarko Petroleum switched it back on in January. Unrefined gas started flowing into an old, 1-inch plastic pipe, called a flow line. It was supposed to be sealed — but it wasn’t.

The unscented gas seeped into the soil near the foundation of the home, filling the house through a drain in the basement.

Lepore says a new state directive orders inspections of these flow lines if they’re within 1,000 feet of an occupied building, “to seek to absolutely minimize any possibility of this happening again.”

In Colorado and many other places, suburbs are expanding into areas that used to be oil and gas fields.

This region in north Thornton, Colo., which includes homes, parks, an elementary school and a Wal-Mart Supercenter, has numerous wells that were drilled between 1972 and 1991, indicated with orange markers. Slide back and forth to see development between 2002 and 2015.

“They’re everywhere,” says Shane Davis, an anti-oil and gas activist who runs a blog called fractivist.org. On a recent tour of Firestone, he points out just how close some of these houses are to wells and khaki-colored oil and gas tanks.

“All of this should be banned in residential areas,” Davis says. “There’s no reason for this in residential areas other than profit.”

Inside Energy finds that over the past several years, 50,000 more people in Colorado now live in areas where there is at least one well per square kilometer.

Colorado and California are the only two states that regulate the flow lines coming out of these wells.

Still, in Colorado a comprehensive state map showing where these lines are does not exist. And while new oil wells must be drilled at least 500 feet from homes, there is no state regulation for how far new homes must be built from existing oil wells.

“That dramatic of a setback, is really, really hard to stomach,” says Gregory Miedema, who heads the Northern Colorado Home Builders Association.

He says contractors are willing to consider new regulations on this, but he warns it could drive up prices. “What they want to make sure is that they don’t price the public out of a new home.”

And there is still a lot of demand for new homes here.

In fact, right behind the site of the home explosion, workers are busy with the construction of a new apartment complex, with nearly 300 units.

Dan Boyce is a Colorado-based reporter withInside Energy, a public media collaboration focusing on America’s energy issues. You can follow him@BoyceDan.